Re: [PATCH 01/11] powerpc/powernv/ioda: Fix ref count for devices with their own PE

From: "Oliver O'Halloran" <oohall@gmail.com>
To: Alistair Popple <alistair@popple.id.au>
Cc: Andrew Donnellan <ajd@linux.ibm.com>,
	Alexey Kardashevskiy <aik@ozlabs.ru>,
	Reza Arbab <arbab@linux.ibm.com>, Greg Kurz <groug@kaod.org>,
	Christophe Lombard <clombard@linux.ibm.com>,
	Andrew Donnellan <andrew.donnellan@au1.ibm.com>,
	Frederic Barrat <fbarrat@linux.ibm.com>,
	linuxppc-dev <linuxppc-dev@lists.ozlabs.org>,
	Alastair D'Silva <alastair@au1.ibm.com>
Subject: Re: [PATCH 01/11] powerpc/powernv/ioda: Fix ref count for devices with their own PE
Date: Mon, 18 Nov 2019 12:24:24 +1100	[thread overview]
Message-ID: <CAOSf1CEzb_Y4NdXGNPYTOxTR5w7OtBUU+VY46CR6Ou5kwEhJqA@mail.gmail.com> (raw)
In-Reply-To: <7553103.1BsQ81z3e0@townsend>

On Mon, Nov 18, 2019 at 12:06 PM Alistair Popple <alistair@popple.id.au> wrote:
>
> On Wednesday, 13 November 2019 4:38:21 AM AEDT Frederic Barrat wrote:
> >
> > However, one question is whether this patch breaks nvlink and if nvlink
> > assumes the devices won’t go away because we explicitly take a reference
> > forever. In npu_dma.c, there are 2 functions which allow to find the GPU
> > associated to a npu device, and vice-versa. Both functions return a
> > pointer to a struct pci_dev, but they don’t take a reference on the
> > device being returned. So that seems dangerous. I’m probably missing
> > something.
> >
> > Alexey, Alistair: what, if anything, guarantees, that the npu or gpu
> > devices stay valid. Is it because we simply don’t provide any means to
> > get rid of them ? Otherwise, don’t we need the callers of
> > pnv_pci_get_gpu_dev() and pnv_pci_get_npu_dev() to worry about reference
> > counting ? I’ve started looking into it and the changes are scary, which
> > explains Greg’s related commit 02c5f5394918b.
>
> To be honest the reference counting looks like it has evolved into something
> quite suspect and I don't think you're missing anything. In practice though we
> likely haven't hit any issues because the original callers didn't store
> references to the pdev which would make the window quite small (although the
> pass through work may have changed that). And as you say there simply wasn't
> any means to get rid of them anyway (EEH, hotplug, etc. has never been
> implemented or supported for GPUs and all sorts of terrible things happen if
> you try).

In other words: leaking a ref is the only safe thing to do.

> The best solution would likely be to review the reference counting and to
> teach callers of get_*_dev() to call pci_put_dev(), etc.

The issue is that the two callers of get_pci_dev() are non-GPL
exported symbols so we don't know what's calling them or what their
expectations are. We be doing whatever makes sense for OpenCAPI and if
that happens to cause a problem for someone else, then they can deal
with it.

Oliver